pls: Fix build with grilo never installed

This re-applies the fix from:
https://git.gnome.org/browse/grilo/commit/?id=3f66bb29a2d0f81479a435c1fabc80c08c4961be

Which got broken by the wrong de-duplicating done in
5f88b535800d1105439641795ddd2e07fd113744

https://bugzilla.gnome.org/show_bug.cgi?id=724871